Activins are dimeric proteins which have the ability to stimulate the production of follicle stimulating hormone (FSH) by the pituitary gland. Activins share a common subunit with inhibins, which inhibit FSH secretion.
Activins are members of a superfamily of polypeptide growth factors which includes the inhibins, the transforming growth factors-.beta. (TGF-.beta.), Mullerian duct inhibiting substance, the Drosophila decapentaplegic, peptide several bone morphogenetic proteins, and the Vg-related peptides.
As a result of their extensive anatomical distribution and multiple biological actions, members of this superfamily of polypeptide growth factors are believed to be involved in the regulation of numerous biological processes. Activin, for example, is involved in the proliferation of many tumor cell lines, the control of secretion and expression of the anterior pituitary hormones (e.g., FSH, GH and ACTH), neuron survival, hypothalamic oxytocin secretion, erythropoiesis, placental and gonadal steroidogenesis, early embryonic development, and the like.
Other members of the activin/TGF-.beta. superfamily of polypeptide growth factors are involved in the regulation of cell function and cell proliferation for numerous cell types, in adults and embryos. For example, cells which are subject to regulation by one or more members of the activin/TGF-.beta. superfamily of polypeptide growth factors include mesenchymal cells, muscle cells, skeletal cells, immune cells, hematopoietic cells, steroidogenic cells, endothelial cells, liver cells, epithelial cells, and the like.
Chemical cross-linking studies with a number of cell types suggests that multiple binding sites (i.e., receptors) exist on the surface of cells. However, little is known about the structure of these receptors, or about the second messenger signalling systems that they employ. It would be desirable, therefore, if the nature of these poorly characterized receptor proteins could be more fully understood.